1. Field of the Invention
The present invention relates generally to the field of radio frequency (R.F.) splitters and combiners and, more particularly, to R.F. splitters and combiners which utilize a .lambda./4 housing to house the .lambda./4 inner conductors.
2. Description of the Prior Art
The capability of being able to split or combine electrical power at radio frequencies, particularly at very high frequencies (VHF) and ultra high frequencies (UHF), is very important. The capability of power splitting is needed when one source or exciter of R.F. power is being used to drive more than one R.F. amplifier or antennas. An ideal power splitter would have a very broad bandwidth and would provide equal amounts of power from the exciter to each active unit connected to each output terminal of the splitter. In the ideal splitter, the shorting of one of the output terminals (i.e., zero impedance) would cause the power delivered by the exciter to each other output terminal to be reduced by an equal amount, thus preventing overdrive of the remaining active units connected to the unshorted output terminals. Moreover, the opening of one of the output terminals (i.e., infinite impedance) would cause the power delivered by the exciter to each other output terminal to remain the same, thus also preventing overdrive of the active units connected to the output terminals.
The ideal splitter would allow the transfer of power from the exciter to each active unit to be independent of the transfer to other active units regardless of load impedance, which would mean that all power reflected by each active unit back towards the exciter (caused by a standing wave ratio of greater than 1:1) would be absorbed in the splitter and would not be propagated or "dumped" to the other active units. In this condition, a very low loss path would exist between the exciter and each active unit, and a very high loss path would exist between each active unit and the exciter and other active units.
An ideal combiner would have a very broad bandwidth and would sum at an output terminal the R.F. power applied at each input terminal, regardless of the level of power applied to and the impedance at each input terminal. In the ideal power combiner, the shorting of one of the input terminals (i.e., zero impedance) would cause the power at the output terminal to be decreased by an amount equal to the level of power applied to that input terminal before shorting, but would not cause any power to be propagated or "dumped" from one input terminal to another input terminal. Moreover, the opening of one of the input terminals (i.e., infinite impedance) would cause the power at the output terminal to be decreased by an amount equal to the level of power applied to that input terminal before opening, but would not cause any power to be "dumped" from one input terminal to another input terminal. Thus, each active unit connected to an input terminal would electrically "see" the output terminal separately and would be unaffected by the operation of the active units connected to the other input terminals.
Presently-known power splitters and combiners exhibit performance capabilities far removed from the ones that would be found in the idealized versions given above. With respect to power splitters, the active unit having the lowest input impedance receives a disproportionate amount of power from the exciter, the amount of which is given by ohm's law. This phenomena of unequal drive power being furnished by the prior art splitters to each active unit results in power "hogging" by the active units having the lowest impedances, which produces several disadvantages.
Because of the different load impedances of the active units, the exciter has problems achieving optimum tuning with respect to the various active units as electrically "seen" through the power splitter. Moreover, in order to provide optimum drive power to the active units having higher impedance loads, the exciter drive power must be increased. However, this increase in exciter drive power often results in an overdrive condition with respect to the lower impedance loads, which at best causes distortion in the output signals and at the worst causes electrical failure to the active units having the lower impedance loads.
Electrical failure to the lower impedance loads is particularly prevalent with amplifiers using solid state devices, as opposed to tubes, as the active elements because solid state devices are currently more susceptible to electrical failure from overdrive when operation is at VHF or UHF. In order to prevent overdrive electrical failure using presently-known power splitters, it has often been necessary to drive all of the active units at a level below that of optimum efficiency in order to be certain to prevent any electrical failures. An alternate approach using prior art power splitters has been to add additional impedance elements to the input stages of each of the active units so as to make the effective input impedance of each active unit equal. However, these impedance elements often must have very low ohmic values and often must have high dissipation and tolerance ratings which cause considerable increased cost. Moreover, considerable time must be spent selecting the correct input impedance value for each active unit, and this process usually must be repeated each time the active device of the active unit is replaced.
An additional problem present in prior art power splitters is caused by the power reflected back to the exciter and the other active units by an active unit that is not optimally matched to the exciter by the splitter. This reflected power causes drive power to be spilled over or "dumped" through the splitter from each active unit to the other active units. When one of the active units becomes an open circuit (infinite impedance), overdrive can suddenly become present with the remaining active units because of the reflected power from the open output terminal. This can cause a type of destructive electrical domino effect which does not stop until the active elements in each of the active units has failed.
Presently-known hybrid and N-way combiners exhibit several major deficiencies. When the output power produced by several active units is summed using present-day combiners, power is "dumped" by each active unit to the others unless each exhibits the same output impedance and power level. This power "dumping" often results in electrical failure to the output active devices in the active units. It should be noted that prior art combiners in such applications often consist of separate 1/4 .lambda. (at the frequency of operation) coaxial lines connected between each active unit and the common output terminal. Each 1/4 .lambda. line has a separate outer braid or sheath. An alternative approach is to use a power combining "tree", which changes the impedance in steps between the input terminals and the output terminals. Such "trees" require a considerable number of stages, which reduces bandwidth and greatly increases attenuation in the forward direction and must be tailor-made for each set of input and output impedances and power levels.